Polyamides are commonly used for the coating of metal substrates, owing especially to their good mechanical properties such as abrasion resistance, impact strength and their chemical inertness towards numerous products such as hydrocarbons, mineral acids and bases. But it is known that the adhesion of polyamides to metals is poor.
To overcome this drawback, the metal support is generally coated with an undercoat, called an adhesion primer, intended to ensure bonding and mechanical anchoring of the polyamide powder. In general the adhesion primer that is used is based on thermosetting resins and is applied in powder form, in solution or in suspension in organic or aqueous solvents. It is necessary therefore to allow for additional facilities, a continuance of the coating operations and therefore a sizeable increase in the production costs. It is increasingly preferred therefore to do without the adhesion primer and to improve the direct adhesion of the coating onto the substrate.
In Patent EP 0 412 288, mixtures of polyamides and epoxy/sulphonamide resins have also been described that can be used to coat metal substrates without this time using an adhesion primer undercoat. These polyamide and epoxy/sulphonamide resin powder mixtures are applied to the substrate using an electrostatic spray gun. Placing the substrate coated in this way into an oven is then enough to melt the powder and obtain a uniform coating. It is also possible to preheat the substrate to above the melting point of the powder and to dip it in a fluidized bed of powder.
In Application PCT/FR/95/01740, pulverulent compositions based on polyamide, on a copolymer of ethylene and unsaturated carboxylic acids or their vinyl or nonvinyl esters and possibly on an ethylene/vinyl alcohol copolymer have been proposed.
The Applicant has now found another polyamide-based powder composition free of adhesion promoter, that can be used to coat metal substrates, and that does not require an adhesion primer layer and that has a very good resistance to salt fog.